1. Field of the Invention
The present invention relates generally to spacecraft and, more particularly, to orbital control of spacecraft.
2. Description of the Related Art
FIG. 1 illustrates a spacecraft 20 in a synchronous orbit 22 (e.g., a geostationary orbit) which defines an orbital plane 24. In order to perform its designed service, the spacecraft 20 is typically required to maintain a controlled spatial relationship with the earth 26.
This relationship is generally fixed with the aid of a station-keeping box 28 which has predetermined dimensions that define the maximum permitted excursions of the spacecraft 20 (particularly in longitude and latitude). The dimensions are determined by various operational and service considerations (e.g., beamwidth of spacecraft antennas, ground station tracking capabilities and specified interference limits between the spacecraft 20 and its neighbor spacecraft) and is on the order of 0.2xc2x0 longitude and latitude (i.e., xcx9c75 kilometers) for spacecraft which use regulated communication frequency bands.
Many spacecraft are restrained to a corresponding station-keeping box for their entire service life but service demands (e.g., military and commercial objectives) of others require that they also be capable of station changing, i.e., they must move from an initial box such as the station-keeping box 28 to one or more subsequent boxes such as the station-keeping box 30. The initial and subsequent boxes are separated by a longitude 32 which may be of considerable magnitude.
In order to carry out its intended service, the spacecraft 20 must also maintain a predetermined service attitude such as one in which antennas of the spacecraft 20 are directed at respective service areas on the earth 26 (e.g., along an antenna boresight 33). Disturbance of the service attitude causes interruption of service and avoiding the loss of expensive service time is a prime objective in the operation of spacecraft.
Although current thruster systems are generally effective for performing spacecraft stationkeeping and attitude control, they are generally less suited for spacecraft station changing. For the latter function, some systems employ high-thrust chemical engines (e.g., bipropellant thrusters) which can effect station changing in acceptable time spans. However, sufficient fuel must be carried by the spacecraft for these high-thrust engines and this reduces the spacecraft""s payload.
Other systems employ low-thrust engines (e.g., ion thrusters) which can operate on electrical energy that is supplied by the spacecraft""s solar cell arrays. In order to effect station changing in acceptable time spans, however, the spacecraft""s attitude must typically be disturbed in order to more effectively align these low-thrust engines and this attitude disturbance causes loss of service time.
Current thruster systems, therefore, generally fail to effectively meet the combined demands of station changing, station keeping, momentum dumping and service-loss minimization.
The present invention is directed to thruster systems that can realize station changing, station keeping and. momentum dumping in spacecraft while minimizing any loss of spacecraft service time. These goals are realized with pairs of E and W thrusters, NE and SE thrusters and NW and SW thrusters whose thrust directions are each defined by respective polar angles xcfx86 and slew angles xcex1.
The E and W thrusters are especially suited for rapid station changing while the remaining thrusters generate normal, tangential and radial thrust components required for station keeping. Because all thrusters are directed through the spacecraft""s center of mass during station changing and station keeping firings, the spacecraft""s attitude is not disturbed and loss of service time is avoided. The thrusters are preferably gimbaled so that they can track spatial changes of the spacecraft""s center of mass or, for momentum dumping, be offset from that center of mass. The normal, tangential and radial thrust components of the thruster systems are arranged so that station changing, station keeping and momentum dumping are still realized after the failure of any thruster pair.
Different thruster embodiments are provided whose slew angles are rotated respectively from anti-nadir and nadir portions of the spacecraft""s yaw axis. Although these systems can be realized with various thruster types, they are particularly suited for ion thrusters.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. dr
FIG. 1 is a perspective view of a spacecraft, a spacecraft orbital plane and initial and subsequent station-keeping boxes that are longitudinally-spaced along the orbital plane;
FIG. 2 is a perspective view of a spacecraft and thruster system of the present invention;
FIGS. 3A and 3B are views respectively of the anti-nadir and top sides of the spacecraft and thruster system of FIG. 2;
FIG. 4 is a perspective view which illustrates spherical coordinates of FIGS. 2, 3A and 3B; and
FIG. 5 is a plot of drift rate in an exemplary station-changing maneuver with the thruster system of FIGS. 2, 3A and 3B.